Multi-frequency e.p.r. studies of Argonne and Illinois sample bank coals

• Published in: Fuel (Guildford) 1990-11, Vol.69 (11), p.1405-1411
• Main Authors: Clarkson, R.B., Wang, Wei, Brown, D.R., Crookham, H.C., Beiford, R.L.
• Format: Article
• Language: English
• Subjects: Applied sciences; coal; Coal and derived products; Energy; Exact sciences and technology; Fuels; maceral; spectroscopy; Structure, chemical and physical properties
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/0016-2361(90)90122-7

Eight coals from the Argonne Premium Coal Sample Program (APCSP), and an Illinois No. 6 coal from the Illinois Coal Basin Sample Program (ICBSP) have been investigated, employing a novel very high frequency (VHF) e.p.r. spectrometer operating at a microwave frequency of about 95 GHz and a magnetic field of 3.4 Tesla. Macerals separated from the ICBSP sample by density gradient centrifugation were also investigated. The high fields used in these experiments provide a 10-fold improvement in the dispersion of electronic Zeeman terms over that provided by conventional X-band instruments (9.5 GHz; 0.34 T), enabling us to resolve spectral features not seen at lower frequencies. Model compounds (perylene(+) and dibenzothiophene(+) radicals) were also studied for comparison purposes. It is proposed that low-field peaks in some of the coal and maceral VHP e.p.r. spectra are the result of spin-orbit coupling of unpaired electrons to heteroatoms, primarily sulphur, and this effect is modelled theoretically employing a simple, two-species model. Preliminary pulsed e.p.r. data on a separated fusinite taken at S-band (3 GHz) are also presented to support the electron exchange model for the lineshape of this maceral.